1. Field of the Invention
This invention relates to an improved liquid-crystal heat valve for actively controlling and regulating heat transfer between two otherwise thermally isolated bodies. This invention, which has various potential applications to be identified hereinafter, is more particularly applicable to free-swimming oceanic divers' diving garments, for better control of the flow of heat between a diver and the ambient water. In diving garments, regulation of the flow of heat between a diver and the water is a life and death issue and continues to be one of the foremost problems facing the diving community. With too little insulation, a diver may experience hypothermia, caused by severe lowering of body core temperature. With too much insulation, the diver may suffer hyperthermia, which is heat stroke. Either condition can result in death, especially when the diver is at 60 to 100 feet of depth and cannot safely return to the surface immediately.
The diving garments of interest here are those which provide a free-swimming diver with some time-limited thermal protection against the cold.
2. Description of the Prior Art
U.S. Pat. No. 4,515,206 to Edward F. Carr documents the occurrence of anomalous ordering and alignment effects of liquid-crystal fluids in the presence of electric and magnetic fields. Reference is made to the Carr patent for much relevant background information.
Providing adequate thermal protection for divers wearing diving suits remains a problem to be reckoned with. Present diving garments are not adaptable to changing thermal conditions. They generally provide only a constant amount of insulation, regardless of the diver's level of physical activity during diving. At present, the most effective of these garments are of the dry-suit type, which means there is an outer waterproof garment and an inner dry suit of relatively thick insulation. The inner garment is inflated with either the breathing mixture or some other gas to control buoyancy, and the gas also serves to prevent loss of insulation capability due to compression with increasing depth of the dive.
This type of garment can predispose a diver to experiencing both hypothermia and hyperthermia during a single dive. The garment can provide too little insulation when he is at rest and too much when he is active. If the diver is very active in a highly insulating garment, he is in danger of heat stroke. Generally, the diver will chose a garment which provides enough thermal insulation to keep him from suffering loss of mental ability during short periods (15 minutes) when he is at rest.
No present Navy inventory suit provides adequate protection for an inactive free swimming diver in 29.degree. F. water. As the amount of insulation increases, the suit becomes more bulky and mobility is reduced. When the diver becomes active, he must work harder in a bulky suit to do almost any job and is thus more susceptible to hyperthermia. Therefore, there is a need for a means and method to control the thermal protection of the garment without using materials other than those which are easily carried by the diver, such as the amount of power held in easily portable batteries.
The use of thermotropic liquid crystals (TLC) for active regulation of heat transfer between the diver and the water can be used to increase diver thermal protection. Thermotropic liquid crystals comprise a class of mesophase materials with variable heat-transfer characteristics. Use of their unique thermal transport properties in a diving garment can offer a significant increase in diver thermal protection. Various methods and means were set forth in the Carr patent. Carr uses liquid-crystal materials and controls their ability in the nematic phase to transfer heat with applied electric fields. He describes two basic methods for electrically regulating the heat transfer across the liquid crystal.
Carr's first method enforces an overall alignment of the liquid-crystal molecules. If the dielectric anisotropy is positive, the molecules will, in general, all align parallel and in the direction of the applied electric field. This general alignment enhances the transfer of heat between the electrodes. If the dielectric anisotropy is negative, all the molecules will, in general, align perpendicular to the field and the flow of heat between the electrodes will be impeded. The fields applied need to be either DC or AC of less than 1 kHz. This prior art method will only vary the thermal conductivity by a small amount, maybe about a factor of two to three.
Carr's second method of thermal regulation uses the electrohydrodynamic (EHD) motion induced by the applied electric field. An applied AC or DC field sets up convective flow cells via EHD motion and can transfer large quantities of heat. The amount of heat thus transferred can be increased by as much as a factor of 50 or more. A more detailed explanation of EHD and flow cells is found in the Carr patent and in Blinov, Electro-Optical and Magneto-Optical Properties of Liquid Crystals, John Wiley & Sons, especially at page 162.
Both of Carr's methods require relatively small amounts of power to control the heat transfer through the liquid-crystal material. For example, 8 mW of power are required for a cell which is 0.66 cm thick and 2 square cm in area to more than double its effective thermal conductivity. However, for large arrays of liquid-crystal cells, this power requirement is not small, especially if a diver needs to carry the batteries. Carr's patent does not disclose how this power is consumed. If it is all consumed in activating the heat-transfer mechanism, then the application of this heat-transferring effect may be limited due to the power expenditure per watt of heat transferred. If the power is mostly consumed by joule heating, then the amount of required power may be significantly reduced by controlling the liquid-crystal electrical resistivity while maintaining an adequate heat transfer capability. This can be accomplished only if the heat transfer behavior is understood or known in more detail than is presented in Carr's patent.
There are also other serious limitations or unknowns in Carr's aforenamed approaches. With the electrodes of his invention in contact with the liquid-crystal material, certain adverse electrochemical reactions occur. These are described in various prior art publications, including Denat et al, Chemical and Electrochemical Stability of p-Methoxybenzilidene-p-n-Butylanaline, Chem. Phys. Lett, vol. 18, page 235 (1973); Briere et al, Correlation between Chemical and Electrochemical Reactivity and DC Conduction in the Isotropic Phase of a Liquid Crystal p-Methoxybenzylidene-p-n-butylaniline, Mol. Cryst. and Liq. Cryst., vol. 19, page 157: and Barret et al, Dynamic Scattering in Nematic Liquid Crystals under DC Conditions, I. Basic electrochemical analysis, Journal Appl. Phys., Vol. 47, No. 6 (1976), which may be incorporated by reference. The rate of the reactions is roughly proportional to the magnitude of the applied electric field and will be larger for DC than for AC fields. These reactions adversely affect the regulation of heat transfer in the affected liquid crystal. Due to certain chemical reactions, liquid-crystal materials can be rapidly degraded and no longer provide regulated heat transfer. The degrading can occur in as little as a few days in 10,000 volts/cm fields.
Carr's methods also use boundary conditions or large magnetic fields (6 kilogauss) to enhance initial alignment conditions and to return the liquid crystal to initial conditions after removal of the controlling voltages. Using boundary conditions to effect liquid-crystal alignment requires the liquid-crystal thickness usually to be limited to less than 100 microns. A thin liquid-crystal cell cannot function as a good thermal insulator. Large magnetic fields will align thicker liquid crystal cells than 100 microns, but substantial hardware is required to generate and power the field. The Carr patent does not deal with changes in the thermal-transfer ability as the temperature of the working material, i.e. the liquid crystal, is varied. The highest increases in ability to transfer heat occur in the convective cell mode where the heat is transferred by movement of the liquid-crystal molecules across the interface between the two bodies. The temperature dependence of viscosity has been found to strongly affect the liquid crystal's capability to transfer heat, as shown in Table 1 hereinafter. At a given frequency and an applied voltage, lowering the liquid crystal's temperature while still remaining within the nematic phase significantly decreases the ability to transfer heat, as shown in FIG. 10.
Additionally, the Carr patent fails to describe or suggest any contemplated liquid-crystal encapsulation, such as the improved heat valve of the type being described in this present invention. Only the generalized connecting of the two bodies by the liquid crystal is discussed in that patent. Although, in general, liquid crystals tend to be inert, chemical reactions and or chemical contamination with nonliquid-crystal materials can occur as discussed in the references cited above. These adverse occurrences can severely degrade the thermal-transfer capabilities. Accordingly, the materials in contact with the liquid crystal must be carefully chosen.
The thermotropic liquid crystals (TLC) can have a large coefficient of thermal expansion. The method of encapsulation, as proposed herein, takes this into account to prevent leakage from and the formation of bubbles in the encapsulated cell.